# coding=utf-8

# This file is part of MyPaint.
# Copyright (C) 2012-2015 by Andrew Chadwick <a.t.chadwick@gmail.com>
#
# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; either version 2 of the License, or
# (at your option) any later version.


"""Color objects and transformation functions.

Color objects are lightweight polymorphic structures which can be cloned and
freely substituted no matter what subtype: they all offer methods for getting
RGB or HSV triples, and can all be constructed from any other type of color
object via a common interface. This gets around some of the UI awkwardnesses of
supporting multiple color spaces: only when you update a central shared color
with an adjuster does its type change to match the control's color space.

"""

## Imports
from __future__ import division, print_function

import re
import colorsys
import struct

import gi
from gi.repository import GdkPixbuf

from lib.helpers import clamp


## Lightweight color objects


class UIColor (object):
    """Base class for color objects which can be manipulated via the UI.

    This base provides a common interface allowing concrete subclasses to be
    instantiated from other UIColors: this provides a mechanism for conversions
    between color models by mixed systems of user interface components as
    needed. Color objects are typically instantiated individually by
    specifying their components, but they may be constructed from other
    `UIColor` objects too:

      >>> col1 = RGBColor(r=0.1, g=0.33, b=0.5)
      >>> col2 = HSVColor(color=col1)

    Subclasses must implement `get_rgb()`, and a ``color`` keyword to
    their constructor which takes as one of its arguments a `UIColor` object.

    """

    def get_rgb(self):
        """Extracts a floating-point R,G,B tuple representation of the color.

        This is unimplemented at this level, but is required by most
        conversions. Subclasses are required to define this function, which
        must return a floating-point ``(r, g, b)`` representation of the color
        with the channel samples lying in the range 0.0 and 1.0 inclusive.

        """
        raise NotImplementedError

    def get_hsv(self):
        """Extracts a floating-point H,S,V tuple representation of the color.

        All terms in the returned ``(h, s, v)`` triple must be scaled to lie in
        the range 0.0 to 1.0 inclusive, compatible with `colorsys`. At this
        level, the operation is defined using an invocation of `get_rgb()`, but
        this behaviour can be overridden by subclasses.

        """
        return colorsys.rgb_to_hsv(*self.get_rgb())

    ## Read-only properties (at this level)
    #rgb = property(get_rgb)
    #hsv = property(get_hsv)

    # RGB read-only
    @property
    def r(self):
        """Read-only RGB red value."""
        return self.get_rgb()[0]

    @property
    def g(self):
        """Read-only RGB green value."""
        return self.get_rgb()[1]

    @property
    def b(self):
        """Read-only RGB blue value."""
        return self.get_rgb()[2]

    # HSV read-only
    @property
    def h(self):
        """Read-only hue angle."""
        return self.get_hsv()[0]

    @property
    def s(self):
        """Read-only HSV saturation."""
        return self.get_hsv()[1]

    @property
    def v(self):
        """Read-only HSV value."""
        return self.get_hsv()[2]

    # Utility methods

    def get_luma(self):
        """Returns a perceptually-weighted brightness suitable for drawing.

          >>> col = RGBColor(0.6, 0.6, 0.6)
          >>> col.get_luma()
          0.6

        The weightings are fixed and rather generic, and the conversion relies
        on a potential RGB conversion. Thus, it's better to use a UIColor
        subclass if this needs to be manipulated. In particular, there's no
        way of just setting the luma defined at this level.

        """
        r, g, b = self.get_rgb()
        return 0.299*r + 0.587*g + 0.114*b

    def to_greyscale(self):
        """Returns a greyscaled version of the color.

          >>> col = RGBColor(r=1.0, g=0.8, b=0.2)
          >>> col = col.to_greyscale()
          >>> min(col.get_rgb()) == max(col.get_rgb())
          True

        Based on `get_luma()`, so the same caveats apply. The returned object
        is itself a `UIColor`.

        """
        luma = self.get_luma()
        return RGBColor(luma, luma, luma)

    def to_contrasting(self, k=0.333):
        """Returns a contrasting `UIColor` suitable for drawing.

          >>> col = RGBColor(r=1.0, g=0.8, b=0.2)
          >>> col != col.to_contrasting()
          True

        """
        luma = self.get_luma()
        c = (luma + k) % 1.0
        return RGBColor(c, c, c)

    def __eq__(self, col):
        """Two base color objects are equal if their RGB form is equal.

        Subclasses should override this with a type-specific alternative
        which compares in a more appropriate fashion. This
        implementation is only intended for comparing between dissimilar
        colour classes.

        """
        # Round to 8bpc for comparison
        rgb1 = [int(c * 0xff) for c in self.get_rgb()]
        try:
            rgb2 = [int(c * 0xff) for c in col.get_rgb()]
        except AttributeError:
            return False
        return rgb1 == rgb2
        # colorhistory.py uses
        #a_ = np.array(helpers.hsv_to_rgb(*a))
        #b_ = np.array(helpers.hsv_to_rgb(*b))
        #return ((a_ - b_)**2).sum() < (3*1.0/256)**2

    def __copy__(self):
        """Clones the object using its own constructor; see `copy.copy()`.
        """
        color_class = type(self)
        return color_class(color=self)

    def __deepcopy__(self, memo):
        """Clones the object using its own constructor; see `copy.deepcopy()`.
        """
        color_class = type(self)
        return color_class(color=self)

    __HEX_PARSE_TABLE = [
        (re.compile('^(?:#|0x)' + '([0-9a-fA-F]{2})' * 3 + '$'), 0xff),
        (re.compile('^(?:#|0x)' + '([0-9a-fA-F])' * 3 + '$'), 0xf),
    ]

    @classmethod
    def new_from_hex_str(class_, hex_str, default=[0.5, 0.5, 0.5]):
        """Construct from an RGB hex string, e.g. ``#ff0000``.
        """
        hex_str = str(hex_str)
        r, g, b = default
        for pr, pd in class_.__HEX_PARSE_TABLE:
            m = pr.match(hex_str)
            if m:
                r, g, b = [float.fromhex(x)/pd for x in m.groups()]
                break
        return RGBColor(r, g, b)

    def to_hex_str(self, prefix='#'):
        """Converts to an RGB hex string of the form ``#RRGGBB``
        """
        r, g, b = [int(c * 0xff) for c in self.get_rgb()]
        return "%s%02x%02x%02x" % (prefix, r, g, b)

    def to_fill_pixel(self):
        """Converts to a pixel value for `Gdk.Pixbuf.fill()`.

          >>> col = RGBColor(1,1,1)
          >>> "%08x" % (col.to_fill_pixel(),)
          'ffffffff'

        """
        r, g, b = [int(c * 0xff) for c in self.get_rgb()]
        pixel = (r << 24) | (g << 16) | (b << 8) | 0xff
        return pixel

    @classmethod
    def new_from_pixbuf_average(class_, pixbuf):
        """Returns the the average of all colors in a pixbuf."""
        assert pixbuf.get_colorspace() == GdkPixbuf.Colorspace.RGB
        assert pixbuf.get_bits_per_sample() == 8
        n_channels = pixbuf.get_n_channels()
        assert n_channels in (3, 4)
        if n_channels == 3:
            assert not pixbuf.get_has_alpha()
        else:
            assert pixbuf.get_has_alpha()
        data = pixbuf.get_pixels()
        w, h = pixbuf.get_width(), pixbuf.get_height()
        rowstride = pixbuf.get_rowstride()
        n_pixels = w*h
        r = g = b = 0
        for y in xrange(h):
            for x in xrange(w):
                offs = y*rowstride + x*n_channels
                r += ord(data[offs])
                g += ord(data[offs+1])
                b += ord(data[offs+2])
        r = r / n_pixels
        g = g / n_pixels
        b = b / n_pixels
        return RGBColor(r/255, g/255, b/255)

    def interpolate(self, other, steps):
        """Generator: interpolate between this color and another."""
        raise NotImplementedError


class RGBColor (UIColor):
    """Additive Red/Green/Blue representation of a color."""

    # Base class overrides: make r,g,b attributes read/write
    r = None
    g = None
    b = None

    def __init__(self, r=None, g=None, b=None, rgb=None, color=None):
        """Initializes from individual values, or another UIColor

          >>> col1 = RGBColor(1, 0, 1)
          >>> col2 = RGBColor(r=1, g=0.0, b=1)
          >>> col1 == col2
          True
          >>> RGBColor(color=HSVColor(0.0, 0.0, 0.5))
          <RGBColor r=0.5000, g=0.5000, b=0.5000>
        """
        UIColor.__init__(self)
        if color is not None:
            r, g, b = color.get_rgb()
        if rgb is not None:
            r, g, b = rgb
        self.r = r  #: Read/write red channel, range 0.0 to 1.0
        self.g = g  #: Read/write green channel, range 0.0 to 1.0
        self.b = b  #: Read/write blue channel, range 0.0 to 1.0
        assert self.r is not None
        assert self.g is not None
        assert self.b is not None

    def get_rgb(self):
        return self.r, self.g, self.b

    def __repr__(self):
        return "<RGBColor r=%0.4f, g=%0.4f, b=%0.4f>" \
            % (self.r, self.g, self.b)

    def interpolate(self, other, steps):
        """RGB interpolation.

        >>> white = RGBColor(r=1, g=1, b=1)
        >>> black = RGBColor(r=0, g=0, b=0)
        >>> [c.to_hex_str() for c in white.interpolate(black, 3)]
        ['#ffffff', '#7f7f7f', '#000000']
        >>> [c.to_hex_str() for c in black.interpolate(white, 3)]
        ['#000000', '#7f7f7f', '#ffffff']

        """
        assert steps >= 3
        other = RGBColor(color=other)
        for step in xrange(steps):
            p = step / (steps - 1)
            r = self.r + (other.r - self.r) * p
            g = self.g + (other.g - self.g) * p
            b = self.b + (other.b - self.b) * p
            yield RGBColor(r=r, g=g, b=b)

    def __eq__(self, other):
        """Equality test (override)

        >>> c1 = RGBColor(0.7, 0.45, 0.55)
        >>> c2 = RGBColor(0.4, 0.55, 0.45)
        >>> c2hcy = HCYColor(color=c2)
        >>> c1 == c2
        False
        >>> c1 == c1 and c2 == c2
        True
        >>> c2 == c2hcy
        True
        >>> c1 == c2hcy
        False

        """
        try:
            t1 = self.get_rgb()
            t2 = other.get_rgb()
        except AttributeError:
            return UIColor.__eq__(self, other)
        else:
            t1 = [round(c, 3) for c in t1]
            t2 = [round(c, 3) for c in t2]
            return t1 == t2


class HSVColor (UIColor):
    """Cylindrical Hue/Saturation/Value representation of a color.

      >>> col = HSVColor(0.6, 0.5, 0.4)
      >>> col.h = 0.7
      >>> col.s = 0.0
      >>> col.v = 0.1
      >>> col.get_rgb()
      (0.1, 0.1, 0.1)

    """

    # Base class overrides: make h,s,v attributes read/write
    h = None
    s = None
    v = None

    def __init__(self, h=None, s=None, v=None, hsv=None, color=None):
        """Initializes from individual values, or another UIColor

          >>> col1 = HSVColor(1.0, 0.5, 0.7)
          >>> col2 = HSVColor(h=1, s=0.5, v=0.7)
          >>> col1 == col2
          True
          >>> HSVColor(color=RGBColor(0.5, 0.5, 0.5))
          <HSVColor h=0.0000, s=0.0000, v=0.5000>
        """
        UIColor.__init__(self)
        if color is not None:
            h, s, v = color.get_hsv()
        if hsv is not None:
            h, s, v = hsv
        self.h = h  #: Read/write hue angle, scaled to the range 0.0 to 1.0
        self.s = s  #: Read/write HSV saturation, 0.0 to 1.0
        self.v = v  #: Read/write HSV value, 0.0 to 1.0
        assert self.h is not None
        assert self.s is not None
        assert self.v is not None

    def get_hsv(self):
        return self.h, self.s, self.v

    def get_rgb(self):
        return colorsys.hsv_to_rgb(self.h, self.s, self.v)

    def __repr__(self):
        return "<HSVColor h=%0.4f, s=%0.4f, v=%0.4f>" \
            % (self.h, self.s, self.v)

    def interpolate(self, other, steps):
        """HSV interpolation, sometimes nicer looking than RGB.

        >>> red_hsv = HSVColor(h=0, s=1, v=1)
        >>> green_hsv = HSVColor(h=1./3, s=1, v=1)
        >>> [c.to_hex_str() for c in green_hsv.interpolate(red_hsv, 3)]
        ['#00ff00', '#ffff00', '#ff0000']
        >>> [c.to_hex_str() for c in red_hsv.interpolate(green_hsv, 3)]
        ['#ff0000', '#ffff00', '#00ff00']

        Note the pure yellow. Interpolations in RGB space are duller looking:

        >>> red_rgb = RGBColor(color=red_hsv)
        >>> [c.to_hex_str() for c in red_rgb.interpolate(green_hsv, 3)]
        ['#ff0000', '#7f7f00', '#00ff00']

        """
        assert steps >= 3
        other = HSVColor(color=other)
        # Calculate the shortest angular distance
        # Normalize first
        ha = self.h % 1.0
        hb = other.h % 1.0
        # If the shortest distance doesn't pass through zero, then
        hdelta = hb - ha
        # But the shortest distance might pass through zero either antilockwise
        # or clockwise. Smallest magnitude wins.
        for hdx0 in -(ha+1-hb), (hb+1-ha):
            if abs(hdx0) < abs(hdelta):
                hdelta = hdx0
        # Interpolate, using shortest angular dist for hue
        for step in xrange(steps):
            p = step / (steps - 1)
            h = (self.h + hdelta * p) % 1.0
            s = self.s + (other.s - self.s) * p
            v = self.v + (other.v - self.v) * p
            yield HSVColor(h=h, s=s, v=v)

    def __eq__(self, other):
        """Equality test (override)

        >>> c1 = HSVColor(0.7, 0.45, 0.55)
        >>> c2 = HSVColor(0.4, 0.55, 0.45)
        >>> c2rgb = RGBColor(color=c2)
        >>> c1 == c2
        False
        >>> c1 == c1 and c2 == c2
        True
        >>> c2 == c2rgb
        True
        >>> c1 == c2rgb
        False

        Colours with zero value but differing hues or saturations must
        test equal. The same isn't true of the other end of the
        cylinder.

        >>> HSVColor(0.7, 0.45, 0.0) == HSVColor(0.4, 0.55, 0.0)
        True
        >>> HSVColor(0.7, 0.45, 1.0) == HSVColor(0.4, 0.55, 1.0)
        False

        """
        try:
            t1 = self.get_hsv()
            t2 = other.get_hsv()
        except AttributeError:
            return UIColor.__eq__(self, other)
        else:
            t1 = [round(c, 3) for c in t1]
            t2 = [round(c, 3) for c in t2]
            if t1[-1] == t2[-1] == 0:
                return True
            return t1 == t2


class HCYColor (UIColor):
    """Cylindrical Hue/Chroma/Luma color, with perceptually weighted luma.

    Not an especially common color space. Sometimes referred to as HSY, HSI,
    or (occasionally and wrongly) as HSL. The Hue `h` term is identical to that
    used by `HSVColor`. Luma `y`, however, is a perceptually-weighted
    representation of the brightness. This ordinarily would make an assymetric
    colorspace solid not unlike the Y'CbCr one because the red, green and blue
    primaries underlying it do not contribute equally to the human perception
    of brightness. Therefore the Chroma `c` term is the fraction of the maximum
    permissible saturation at the given `h` and `y`: this scaling to within the
    legal RGB gamut causes the resultant color space to be a regular cylinder.

    In practical terms, adjusting luma alone moves the color along a shading
    series of uniform relative saturation towards either white or black. This
    feature is useful for gamut masking especially, and when working in
    painting styles where value is drawn first and color applied later.
    However the pure "digital" colors appear at different heights in the
    color solid of this model, which can be confusing.

    """

    # Base class override: make h attribute read/write
    h = None

    def __init__(self, h=None, c=None, y=None, hcy=None, color=None):
        """Initializes from individual values, or another UIColor

          >>> col1 = HCYColor(0, 0.1, 0.2)
          >>> col2 = HCYColor(h=0, c=0.1, y=.2)
          >>> col3 = HCYColor(hcy=[0, 0.1, .2])
          >>> col1 == col2 and col2 == col3
          True
          >>> HCYColor(color=RGBColor(0.5, 0.5, 0.5))
          <HCYColor h=0.0000, c=0.0000, y=0.5000>
        """
        UIColor.__init__(self)
        if color is not None:
            if isinstance(color, HCYColor):
                h = color.h
                c = color.c
                y = color.y
            else:
                h, s, v = color.get_hsv()
                h_, c, y = RGB_to_HCY(colorsys.hsv_to_rgb(h, s, v))
        if hcy is not None:
            h, c, y = hcy
        self.h = h  #: Read/write hue angle, scaled to the range 0.0 to 1.0
        self.c = c  #: Read/write HCY chroma, 0.0 to 1.0
        self.y = y  #: Read/write HCY luma, 0.0 to 1.0
        assert self.h is not None
        assert self.c is not None
        assert self.y is not None

    def get_hsv(self):
        rgb = self.get_rgb()
        h, s, v = colorsys.rgb_to_hsv(*rgb)
        return self.h, s, v

    def get_rgb(self):
        return HCY_to_RGB((self.h, self.c, self.y))

    def get_luma(self):
        return self.y

    def __repr__(self):
        return "<HCYColor h=%0.4f, c=%0.4f, y=%0.4f>" \
            % (self.h, self.c, self.y)

    def interpolate(self, other, steps):
        """HCY interpolation.

        >>> red = HCYColor(0, 0.8, 0.5)
        >>> green = HCYColor(1./3, 0.8, 0.5)
        >>> [c.to_hex_str() for c in green.interpolate(red, 5)]
        ['#19c619', '#5ea319', '#8c8c19', '#c46f19', '#e55353']
        >>> [c.to_hex_str() for c in red.interpolate(green, 5)]
        ['#e55353', '#c46f19', '#8c8c19', '#5ea319', '#19c619']

        HCY is a cylindrical space, so interpolations between two endpoints of
        the same chroma will preserve that chroma. RGB interpoloation tends to
        diminish because the interpolation will pass near the diagonal of zero
        chroma.

        >>> [i.c for i in red.interpolate(green, 5)]
        [0.8, 0.8, 0.8, 0.8, 0.8]
        >>> red_rgb = RGBColor(color=red)
        >>> [round(HCYColor(color=i).c, 3)
        ...       for i in red_rgb.interpolate(green, 5)]
        [0.8, 0.457, 0.571, 0.686, 0.8]

        """
        assert steps >= 3
        other = HCYColor(color=other)
        # Like HSV, interpolate using the shortest angular distance.
        ha = self.h % 1.0
        hb = other.h % 1.0
        hdelta = hb - ha
        for hdx0 in -(ha+1-hb), (hb+1-ha):
            if abs(hdx0) < abs(hdelta):
                hdelta = hdx0
        for step in xrange(steps):
            p = step / (steps - 1)
            h = (self.h + hdelta * p) % 1.0
            c = self.c + (other.c - self.c) * p
            y = self.y + (other.y - self.y) * p
            yield HCYColor(h=h, c=c, y=y)

    def __eq__(self, other):
        """Equality test (override)

        >>> c1 = HCYColor(0.7, 0.45, 0.55)
        >>> c2 = HCYColor(0.4, 0.55, 0.45)
        >>> c2rgb = RGBColor(color=c2)
        >>> c1 == c2
        False
        >>> c1 == c1 and c2 == c2
        True
        >>> c2rgb == c2
        True
        >>> c1 == c2rgb
        False

        Two colours with identical lumas but with differing hues or
        saturations must test equal if their luma is pure black or pure
        white.

        >>> HCYColor(0.7, 0.45, 0.0) == HCYColor(0.4, 0.55, 0.0)
        True
        >>> HCYColor(0.7, 0.45, 1.0) == HCYColor(0.4, 0.55, 1.0)
        True

        """
        try:
            t1 = (self.h, self.c, self.y)
            t2 = (other.h, other.c, other.y)
        except AttributeError:
            return UIColor.__eq__(self, other)
        else:
            t1 = [round(c, 3) for c in t1]
            t2 = [round(c, 3) for c in t2]
            if t1[-1] == t2[-1] == 0:
                return True
            if t1[-1] == t2[-1] == 1:
                return True
            return t1 == t2

class YCbCrColor (UIColor):
    """YUV-type color, using the BT601 definition.

    This implementation uses the BT601 Y'CbCr definition. Luma (`Y`) ranges
    from 0 to 1, the chroma components (`Cb` and `Cr`) range from -0.5 to 0.5.
    The projection of this space onto the Y=0 plane is similar to a slightly
    tilted regular hexagon.

    This color space is derived from the displayable RGB space. The luma or
    chroma components may be manipluated, but because the envelope of the RGB
    cube does not align with this space's axes it's quite easy to go out of
    the displayable gamut.

    """

    def __init__(self, Y=None, Cb=None, Cr=None, YCbCr=None, color=None):
        """Initializes from individual values, or another UIColor"""
        UIColor.__init__(self)
        if color is not None:
            if isinstance(color, YCbCrColor):
                Y = color.Y
                Cb = color.Cb
                Cr = color.Cr
            else:
                rgb = color.get_rgb()
                Y, Cb, Cr = RGB_to_YCbCr_BT601(rgb)
        if YCbCr is not None:
            Y, Cb, Cr = YCbCr
        self.Y = Y  #: Read/write BT601 luma, 0.0 to 1.0
        self.Cb = Cb  #: Read/write BT601 blue-difference chroma, -0.5 to 0.5.
        self.Cr = Cr  #: Read/write BT601 red-difference chroma, -0.5 to 0.5.
        assert self.Y is not None
        assert self.Cb is not None
        assert self.Cr is not None

    def get_luma(self):
        return self.Y

    def get_rgb(self):
        """Gets a raw RGB triple, possibly out of gamut.
        """
        return YCbCr_to_RGB_BT601((self.Y, self.Cb, self.Cr))

    def __repr__(self):
        return "<YCbCrColor Y=%0.4f, Cb=%0.4f, Cr=%0.4f>" \
            % (self.Y, self.Cb, self.Cr)

    def interpolate(self, other, steps):
        """YCbCr interpolation.

        >>> yellow = YCbCrColor(color=RGBColor(1,1,0))
        >>> red = YCbCrColor(color=RGBColor(1,0,0))
        >>> [c.to_hex_str() for c in yellow.interpolate(red, 3)]
        ['#feff00', '#ff7f00', '#ff0000']

        This colorspace is a simple transformation of the RGB cube, so to
        within a small margin of error, the results of this interpolation are
        identical to an interpolation in RGB space.

        >>> y_rgb = RGBColor(1,1,0)
        >>> r_rgb = RGBColor(1,0,0)
        >>> [c.to_hex_str() for c in y_rgb.interpolate(r_rgb, 3)]
        ['#ffff00', '#ff7f00', '#ff0000']

        """
        assert steps >= 3
        other = YCbCrColor(color=other)
        # Like HSV, interpolate using the shortest angular distance.
        for step in xrange(steps):
            p = step / (steps - 1)
            Y = self.Y + (other.Y - self.Y) * p
            Cb = self.Cb + (other.Cb - self.Cb) * p
            Cr = self.Cr + (other.Cr - self.Cr) * p
            yield YCbCrColor(Y=Y, Cb=Cb, Cr=Cr)

    def __eq__(self, other):
        """Equality test (override)

        >>> c1 = YCbCrColor(0.7, 0.45, 0.55)
        >>> c2 = YCbCrColor(0.4, 0.55, 0.45)
        >>> c2rgb = RGBColor(color=c2)
        >>> c1 == c2
        False
        >>> c1 == c1 and c2 == c2
        True
        >>> c2 == c2rgb
        True
        >>> c1 == c2rgb
        False

        """
        try:
            t1 = (self.Y, self.Cb, self.Cr)
            t2 = (other.Y, other.Cb, other.Cr)
        except AttributeError:
            return UIColor.__eq__(self, other)
        else:
            t1 = [round(c, 3) for c in t1]
            t2 = [round(c, 3) for c in t2]
            return t1 == t2


## ITU.BT-601 Y'CbCr renormalized values (Cb, Cr between -0.5 and 0.5).

# A YCC space, i.e. one luma dimension and two orthogonal chroma axes derived
# directly from an RGB model. Planes of constant Y are roughly equiluminant,
# but the color solid is asymmetrical.
#
# Of marginal interest, the projection of the pure-tone {R,Y,G,C,B,M} onto the
# Y=0 plane is very close to exactly hexagonal. Shame that cross-sections of
# the color solid are irregular triangles, rectangles and pentagons following
# a rectangular cuboid standing on a point.
#
# ref http://www.itu.int/rec/R-REC-BT.601/en


def RGB_to_YCbCr_BT601(rgb):
    """RGB → BT601 YCbCr: R,G,B,Y ∈ [0, 1]; Cb,Cr ∈ [-0.5, 0.5]"""
    R, G, B = rgb
    Y = 0.299 * R + 0.587 * G + 0.114 * B
    Cb = -0.169 * R - 0.331 * G + 0.500 * B
    Cr = 0.500 * R - 0.419 * G - 0.081 * B
    return Y, Cb, Cr


def YCbCr_to_RGB_BT601(YCbCr):
    """BT601 YCbCr → RGB: R,G,B,Y ∈ [0, 1]; Cb,Cr ∈ [-0.5, 0.5]"""
    Y, U, V = YCbCr
    R = Y + 1.403 * V
    G = Y - 0.344 * U - 0.714 * V
    B = Y + 1.773 * U
    return R, G, B


## HCY color space.

# Frequently referred to as HSY, Hue/Chroma/Luma, HsY, HSI etc.  It can be
# thought of as a cylindrical remapping of the YCbCr solid: the "C" term is the
# proportion of the maximum permissible chroma within the RGB gamut at a given
# hue and luma. Planes of constant Y are equiluminant.
#
# ref https://code.google.com/p/colour-space-viewer/
# ref git://anongit.kde.org/kdelibs in kdeui/colors/kcolorspaces.cpp
# ref http://blog.publicfields.net/2011/12/rgb-hue-saturation-luma.html
# ref Joblove G.H., Greenberg D., Color spaces for computer graphics.
# ref http://www.cs.rit.edu/~ncs/color/t_convert.html
# ref http://en.literateprograms.org/RGB_to_HSV_color_space_conversion_(C)
# ref http://lodev.org/cgtutor/color.html
# ref Levkowitz H., Herman G.T., "GLHS: a generalized lightness, hue, and
#     saturation color model"

# For consistency, use the same weights that the Color and Luminosity layer
# blend modes use, as also used by brushlib's Colorize brush blend mode. We
# follow http://www.w3.org/TR/compositing/ here. BT.601 YCbCr has a nearly
# identical definition of luma.

_HCY_RED_LUMA = 0.3
_HCY_GREEN_LUMA = 0.59
_HCY_BLUE_LUMA = 0.11


def RGB_to_HCY(rgb):
    """RGB → HCY: R,G,B,H,C,Y ∈ [0, 1]

    :param rgb: Color expressed as an additive RGB triple.
    :type rgb: tuple (r, g, b) where 0≤r≤1, 0≤g≤1, 0≤b≤1.
    :rtype: tuple (h, c, y) where 0≤h<1, but 0≤c≤2 and 0≤y≤1.

    """
    r, g, b = rgb

    # Luma is just a weighted sum of the three components.
    y = _HCY_RED_LUMA*r + _HCY_GREEN_LUMA*g + _HCY_BLUE_LUMA*b

    # Hue. First pick a sector based on the greatest RGB component, then add
    # the scaled difference of the other two RGB components.
    p = max(r, g, b)
    n = min(r, g, b)
    d = p - n   # An absolute measure of chroma: only used for scaling.
    if n == p:
        h = 0.0
    elif p == r:
        h = (g - b)/d
        if h < 0:
            h += 6.0
    elif p == g:
        h = ((b - r)/d) + 2.0
    else:  # p==b
        h = ((r - g)/d) + 4.0
    h /= 6.0

    # Chroma, relative to the RGB gamut envelope.
    if r == g == b:
        # Avoid a division by zero for the achromatic case.
        c = 0.0
    else:
        # For the derivation, see the GLHS paper.
        c = max((y-n)/y, (p-y)/(1-y))
    return h, c, y


def HCY_to_RGB(hcy):
    """HCY → RGB: R,G,B,H,C,Y ∈ [0, 1]

    :param hcy: Color expressed as a Hue/relative-Chroma/Luma triple.
    :type hcy: tuple (h, c, y) where 0≤h<1, but 0≤c≤2 and 0≤y≤1.
    :rtype: tuple (r, g, b) where 0≤r≤1, 0≤g≤1, 0≤b≤1.

    >>> n = 32
    >>> diffs = [sum( [abs(c1-c2) for c1, c2 in
    ...                zip( HCY_to_RGB(RGB_to_HCY([r/n, g/n, b/n])),
    ...                     [r/n, g/n, b/n] ) ] )
    ...          for r in range(int(n+1))
    ...            for g in range(int(n+1))
    ...              for b in range(int(n+1))]
    >>> sum(diffs) < n*1e-6
    True

    """
    h, c, y = hcy

    if c == 0:
        return y, y, y

    h %= 1.0
    h *= 6.0
    if h < 1:
        #implies (p==r and h==(g-b)/d and g>=b)
        th = h
        tm = _HCY_RED_LUMA + _HCY_GREEN_LUMA * th
    elif h < 2:
        #implies (p==g and h==((b-r)/d)+2.0 and b<r)
        th = 2.0 - h
        tm = _HCY_GREEN_LUMA + _HCY_RED_LUMA * th
    elif h < 3:
        #implies (p==g and h==((b-r)/d)+2.0 and b>=g)
        th = h - 2.0
        tm = _HCY_GREEN_LUMA + _HCY_BLUE_LUMA * th
    elif h < 4:
        #implies (p==b and h==((r-g)/d)+4.0 and r<g)
        th = 4.0 - h
        tm = _HCY_BLUE_LUMA + _HCY_GREEN_LUMA * th
    elif h < 5:
        #implies (p==b and h==((r-g)/d)+4.0 and r>=g)
        th = h - 4.0
        tm = _HCY_BLUE_LUMA + _HCY_RED_LUMA * th
    else:
        #implies (p==r and h==(g-b)/d and g<b)
        th = 6.0 - h
        tm = _HCY_RED_LUMA + _HCY_BLUE_LUMA * th

    # Calculate the RGB components in sorted order
    if tm >= y:
        p = y + y*c*(1-tm)/tm
        o = y + y*c*(th-tm)/tm
        n = y - (y*c)
    else:
        p = y + (1-y)*c
        o = y + (1-y)*c*(th-tm)/(1-tm)
        n = y - (1-y)*c*tm/(1-tm)

    # Back to RGB order
    if h < 1:
        return (p, o, n)
    elif h < 2:
        return (o, p, n)
    elif h < 3:
        return (n, p, o)
    elif h < 4:
        return (n, o, p)
    elif h < 5:
        return (o, n, p)
    else:
        return (p, n, o)


## Module testing

def _test():
    """Run all doctests in this module"""
    import doctest
    doctest.testmod()


if __name__ == '__main__':
    _test()
